1. Field of the Invention
The present invention relates to a tunnel junction type Josephson device and a method for fabricating the same, and more specifically to a novel tunnel junction type Josephson device which utilizes an excellent property of compound oxide superconductor material and a method for fabricating the same.
2. Description of Related Art
Josephson devices have been constituted of a weak link or junction between a pair of superconductors. In order to form the weak link, some constructions have been conventionally proposed. The most typical construction is a tunnel junction type Josephson device, in which an extremely thin insulating layer is interposed between a pair of superconductors.
In tunnel junction type Josephson devices which have been actually fabricated in the prior art, a superconductor has been formed of Nb, NbN, Pb and others, and an insulator layer is formed of an oxide film formed on a surface of a Nb or Pb superconductor, or a deposited film of MgO or a--Si. However, these metal type superconductors generally have a very low critical temperature of superconductivity, and therefore, cannot effectively exhibit its superconduction characteristics unless they are cooled by using a greatly expensive liquid helium.
In 1986, on the other hand, it was discovered that a sintered compound oxide such as (La, Ba).sub.2 CuO.sub.4 and (La, Sr).sub.2 CuO.sub.4 is a superconduction material having a high critical temperature Tc. Following on this discovery, it has been found that a compound oxide having a composition expressed by Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x presents an effective superconductivity in a temperature range not less than a liquid nitrogen temperature. The superconductor material capable of assuming the superconductivity in a temperature range not less than a liquid nitrogen temperature. The superconductor material capable of assuming the superconductivity at a high temperature as mentioned above makes it possible to use an inexpensive liquid nitrogen as a cooling agent, and therefore, an actual application of a superconduction technology has been studied. Under this circumstance, various attempts are made in order to realize a Josephson device formed of compound oxide superconductor materials.
However, it has been known that conventional Josephson devices formed as an experiment by using the above mentioned compound oxide superconductor materials did not exhibit a satisfactory characteristics even at 4.2K which is a temperature of the liquid helium.